Men often enlisted in a company recruited in the counties where they lived though not always. After many battles, companies might be combined because so many men were killed or wounded. However if you are unsure which company your ancestor was in, try the company recruited in his county first.
Company A (Captain John B. Thompson) - many men from Scott County
Company B (Captain Samuel P. "Pat" Larmer) - many men from Lee and Scott County
Company C (Captain James S. Collings) - many men from Scott County
Company D (Captain Francis M. Smith) - many men from Scott County
Company E (Captain John T. Radford) (replaced by Captain George G. Junkin) - many men from Montgomery County
Company F (Captain George W. Thomas) - many men from Roanoke, Carroll, Floyd and Montgomery Counties
Company G (Lee Rangers, Captain Thomas S. Gibson) - many men from Lee County
Company H (Captain Samuel P. Larmer) - many men from Lee County
Company I (Lee Rangers) (later Lee Greys) - many men from Lee County and a few from Claiborne and Hancock
The information above is from ''25th Virginia Cavalry'', by Dobbie E. Lambert.
:[http://ranger95.com/civil_war/virginia/cavalry/roster2/25th_cav_rgt_rost_fld_staff.html Field & Staff]